Rf-source resistance meters

ABSTRACT

Several embodiments of RF source resistance measuring devices are disclosed. Common to all embodiments is the feature of the inclusion of at least one variable resistor, and a peak readout meter. In one embodiment, two ganged unloaded potentiometers are employed while another embodiment comprises an automatic-nulling RF power bridge circuit with a variable rather than a fixed bridge reference resistance. A third embodiment comprises a calorimeter with a variable rather than a fixed resistor, while in another embodiment attenuator pads with variable resistors are employed.

O Uted States Patent 1 [111 3,750,016

Low et a1. [4 July 31, 1973 RF-SOURCE RESISTANCE METERS 2,316,153 4/1943Brown 324/57 R 76 Inventors: Geor e M. Low Actin l Admignistrator theNgational Primary Examiner-Rudolph V. Rolinec Aeronautics and SpaceAssistant Examiner--Emest F. Karlsen Administration with respect to theAttorney-Monte F. Mott, Paul F. McCaul and John R. invention of; ErnestC. Oakley, Mannmg Pasadena, Calif. 22 Filed: Feb. 19, 1971 [57] ABSTRACTSeveral embodiments of RF source resistance measur- [21] APP! 116,790ing devices are disclosed. Common to all embodiments is the feature ofthe inclusion of at least one variable re- 52 us. Cl. 324 57 R, 324/295,324/62 R, siswr, and a P readout meter- In one embodiment, 324/95 twoganged unloaded potentiometers are employed 51 Int. Cl. G0lr 27/00, GOlr27 42 While another embodiment comprises an automak- [58] Field ofSearch 324/62 R, 57 R, 29.5, ulling RF Power bridge circuit with avariable rather 324 10 95 4 103 R, 0 p 2 143 64 than a fixed bridgereference resistance. A third embodiment comprises a calorimeter with avariable 56 References Cited rather than a fixed resistor, while inanother embodi- UNITED STATES PATENTS ment attenuator pads with variableresistors are em- 1 2,084,086 6/1937 Heyer 324/295 p oyed 1,520,86512/1924 Heyer 324/295 3 Claims, 9 Drawing Figures PAIENIEB JUL 3 I I975sum 1 or 2 FIG. 3 GENERATOR A A VOLTMETER- RS DETECTOR RZQ KN \L A i 5K?I9 '50 I8 FEEDBACK M TUNEDAF. PRIOR ART RR AMPLIFIER J2 zoon Q J] IOKHZ25 I if ZERO ERNEST c. OAKLEY I N VEN TOR.

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sum 2 or 2 FEEDBACK TUNED A.F. l I R R AMPLIFIER J2 For: I

25 H) I IOKHZ ZERO D.C. BIAS I i P BOLOMETER i I SUP LY J MOUNT Fl 7 IREADOUT METER Rs vvvvm w w 6v; FIG. 9

Fl G. 8 52 RS 1 ERNEST c. OAKLEY INVENTOR. ZIN BY ATTORNEYS RF-SOURCERESISTANCE METERS ORIGIN OF THE INVENTION The invention described hereinwas made in the performance of work under a NASA contract and is subjectto the provisions of Section 305 of the National Aeronautics and SpaceAct of 1958, Public Law 85568 (72 Stat. 435; 42 USC 2457).

BACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to means for measuring the resistance of a radio frequency (RF)source and, more particularly, to improvements therein.

2. Description of the Prior Art There are several known methods ordevices to determine RF source resistance. However, most of these sufferfrom one or more significant disadvantages. A technique known as doubleloading, in which the ratio of the change in output voltage as the loadresistance is changed, is cumbersome, time consuming and is subject toreactive errors beginning in the low VHF region. Also, in practicingsuch a technique normal circuit operation is distrubed. In anothermethod, known as reactive-transformation power peaking, .a coaxial lineor a lumped constant reactive tuner is required, to maximize the powerinto a load resistance. The combined network of the reactive-tuner andthe resistive power detecting device are then disconnected from theoutput port being measured and the conjugate complex impedance of thisnetwork is inferred as the output port parameters. Although such ameasurement produces accurate results, its primary disadvantages are thebulkiness of the coaxial reactive tuners, and the fact that such tunersare not available below the UHF region. There are other RF sourceresistance measurement techniques which are quite sophisticated andcomplex. Most of them treat the non-linear active output port as alinear, passive input port, and hence do not yield a valid or meaningfulmeasurement under normal'circuit operating conditions.

OBJECTS AND SUMMARY OF THE INVENTION It is a primary object of thepresent invention to provide new devices for measuring RF sourceresistance.

Another object of the present invention is to provide novel RF sourceresistance measuring devices which do not suffer from the disadvantagescharacteristic of prior art devices.

A further object of the present invention is to provide RF sourceresistance measuring devices, which are reliable, very simple to operateand relatively inexpensive as compared with prior art devices.

These and other objects of the present invention are achieved byproviding RF source resistance measuring devices, each one of whichincludes at least one variable resistor whose resistance is adjusted toprovide a peak reading on a readout meter, the dial of which iscalibrated in terms of RF source resistance. In one embodiment of theinvention, an automatic-nulling RF power bridge circuit is employed witha variable rather than a fixed bridge reference resistance, and with abolometer mount connected as one leg of the bridge. In anotherembodiment ganged, unloaded potentiometers are used to determine RFsource resistance, while in yet another embodiment the device comprisesa calorimeter with a variable resistor which is adjusted for maximumpower transfer which is recorded by the calorimeter. In yet anotherembodiment of the device, the impedance-matching property of anattenuator pad with variable resistors is utilized. The resistors arevaried for maximum power transfer to the pad which acts as the load whenmaximum power transfer takes place as indicated by an appropriate peakreadout meter, the pad or load resistance matches the RF sourceresistance.

The novel features of the invention are set forth with particularity inthe appended claims. The invention will be best understood from thefollowing description when read in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1-4 are schematic diagramsuseful in explaining embodiments of the invention employing gangedphased potentiometers;

FIG. 5 is a schematic diagram of a prior art automatic-nulling RF powerbridge circuit;

FIG. 6 is a schematic diagram of another embodiment of the invention;and

FIGS. 7-9 are diagrams useful in explaining yet other embodiments of theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The embodiment of the inventionin which ganged unloaded variable resistors or potentiometers areemployed for source resistance measurement may best be explained inconjunction with FIGS. 1-4. In FIG. 1, a potentiometer R, is shownconnected across a voltage source E at terminals 1 l and 12. A secondpotentiometer R, is shown connected across the wiper of R, and tenninal12. The output voltage E is taken across the wiper of R and terminal 12.R is assumed to be much greater than R, (R R,) so as not to appreciablyload R11 If the two potentiometers are ganged and their wipers phasedand tracked as represented by dashed line 14 and arrows 15 and 16, theoutput voltage will vary as the square of the coefficient of rotation(wiper movement). The potentiometers thus ganged perfrom the analogcomputation of solving for the product of the input voltage times thesquare of the percentage of mechanical wiper shaft rotation.

In practice, the potentiometers are connected as shown in FIG. 2.Therein, dashed line 17 encloses a source E,- with an internalresistance R which is to be measured. Relating FIG. 2 to FIG. 1, R, andR are connected across the output terminals 11 and 12 of the source Ewhile the wiper of R, is connected to terminal 12. A voltmeter 18serving as peak readout detector is connected across the wiper of R, andterminal 12. In FIG. 2, the internal resistance R represents the toppart of R, in FIG. 1. As can be seen, R, still divides the voltagedivided by the ratio of the source resistance R and the lower armcomposed of the unshorted part of RI:

The output voltage as indicated on the voltmeter detector 18 varieslinearly as the source resistance R assuming the potentiometers R, andR, static. If however, the source resistance R is held constant and theganged-potentiometers R, and R are varied, a peak reading will beindicated at a discrete se'ttting of the ganged-phased potentiometerpair which will be different for each incremental change of R Acalibration can be established to relate the source resistance R and thepercentage of rotation, from which a direct indication of R isdeterminable.

The value of R can be chosen so that a match to the source resistancewill coincide with a peak reading on the voltmeter-detector 18. Matchwill occur at this value of R since incremental changes in R as it isrotated will vary roughly as the square root of the incremental changesof R AR z VKR RIG. 3 is a schematic diagram of an example of sourceresistance measuring arrangement for a coaxial system with a 50 ohmoperational resistance. R is chosen to be 150 ohm, based on experimentaldata showing R, to lie between 100 and 250 ohms. R is chosen to be 5kohms to yield one-half percent loading error on the terminated networkof R and R at match (25 ohms equivalent). The source resistance as seenby the voltmeter-detector 18 is a maximum of about l.3k ohms. Thevoltmeter-detector 18 is chosen to have a frequency response suitablefor the frequency of the generator source 19. Also, its input resistanceshould be sufficiently high so as not to load R That is, R,- l.3k ohms.It is further desirable that the scale of voltmeter-detector 18 havesufficient resolution to impart ease in determining the peak reading.

FIG. 4 to which reference is now made represents an alternate embodimentin which current through the variable load resistance, rather thanvoltage, is sensed to determine source resistance. In this embodiment,two potentiometers R and R are connected in series between terminals 11and 12 and their wipers are phased and tracked. The resistance value ofR is made low with respect to the resistance of R in order not to limitthe current when R is a relatively small value.

Another embodiment of the present invention employs a bolometer mount inwhich the mount load resistance is varied to provide an indication ofsource resistance. For an understanding of this aspect of the invention,reference is first made to FIG. 5 which represents a schematic diagramof a known RF power bridge circuit. In the bridge 20, one arm is formedby a bolometer mount 22 which is assumed to be connected at terminal J lto an RF power source. The operation of such a circuit for powermeasurements is well known. In actuality, FIG. 5 is the schematicdiagram of a power meter model 4300 manufactured by Hewlett Packard.

It has been discovered that by replacing the fixed bridge referenceresistor R with a variable resistor or potentiometer R as shown in FIG.6, the nominal operating resistance of the power detecting coaxialbolometer mount can be deliberately varied so as to match the mount loadresistance, into which maximum power from the RF source connected to J 1will be dissipated. From this the value of the output resistance of theRF source is directly determinable. This is because the maximum powertransfer occurs when the load re zero when the RF source is disconnectedfrom J1. From the foregoing it is thus seen that by replacing the fixedR of the bridge with a variable resistor R a presently availablebolometeric automatic-nulling RF power meter is provided with theadditional capability of measuring RF source resistance.

in yet another embodiment of the present invention,

source resistance measurement is performed by means of a calorimetercontaining a variable resistor, which is connected across the source,whose resistance is to be measured. Such an arrangement is shown in FIG.7. Therein, the calorimeter is designated by numeral 30, the variableresistor by R, and readout meter, which is assumed to indicate the poweror heat absorbed by'the calorimeter matter such as fluid 32, isdesignated by numeral 34. It should be apparent that maximum powertransfer to fluid 32 occurs when the resistance of R, equals R Thus, thesetting of R, when a peak reading is observed on meter 34 is a directindication of the source resistance R Such a calorimeter sourceresistance measuring device has the following advantages:

1. It is inherently broad-band in frequency;

2. It is simple in design and is easily fabricated;

, 3. It can determine source-resistance over a broad range of inputpower; 4. It is rugged; and 5. It is extremely simple to operate,requiring only an adjust-to-peak adjustment, with direct readout ofsource resistance on a properly calibrated dial. in still anotherembodiment of the invention, source resistance is determined by means oftaper attenuator pads which are either of the Tee or Pi configuration,as shown in FIGS. 8 and 9 wherein they are designated by numerals 52 and54, respectively. Each of the pads may consist of ganged variableresistors rather than the fixed resistors of typical attenuator pads.Since the power delivered to the load (in the fonn of the pad) is amaximum when the load resistance equals the source resistance, bymonitoring a peak power reading in meter 50, which is assumed to be apower meter, the interpolated value of the input impedance Z, of thetapered pad which yields maximum power transfer is the value of thesource resistance. i

There has accordingly been shown and described herein severalembodiments of a source resistance measuring device. In each embodiment,the device includes at least one variable resistor which is adjusted toprovide a peak power reading, with the resistors setting for peak powerbeing used to indicate the resistance of the source to which the deviceis connected. It is appreciated that modifications and variations mayreadily occur to those skilled in the art and, consequently, it isintended that the claims be interpreted to cover such modifications andequivalents.

What is claimed is:

l. A device for measuring the resistance of a source comprising:

a first variable resistor having first and second ends connected acrosssaid source, said first variable resistor having a rotatable wiper arrn,connected to the second end of said first resistor;

a second variable resistor having first and second ends connectedrespectively to the first and second ends of said first variableresistor, said second resistor having a rotatable wiper arm, said wiperarms being ganged and phased for simultaneous rotation tional positionsof the wiper arms and the resistance of said source. 2. A device asrecited in claim 1 wherein said readout means is a voltmeter and saidsecond resistor is larger than said first resistor.

3. A device as recited in claim 2 wherein said source is aradio-frequency source and said voltmeter has a frequency responseincluding the frequency of said source.

I t It i

1. A device for measuring the resistance of a source comprising: a firstvariable resistor having first and second ends connected across saidsource, said first variable resistor having a rotatable wiper arm,connected to the second end of said first resistor; a second variableresistor having first and second ends connected respectively to thefirst and second ends of said first variable resistor, said secondresistor having a rotatable wiper arm, said wiper arms being ganged andphased for simultaneous rotation whereby at each rotational position theratio of the resistance between the second end of said first resistorand its wiper arm in the absence of the connection therebetween to thetotal resistance of said first resistor is equal to the ratio of theresistance between the second end of said second resistor and its wiperarm to the total resistance of said second resistor; and readout meansconnected across the wiper arm of said second resistor and the secondend thereof for providing a peak reading as a function of the rotationalpositions of the wiper arms and the resistance of said source.
 2. Adevice as recited in claim 1 wherein said readout means is a voltmeterand said second resistor is larger than said first resistor.
 3. A deviceas recited in claim 2 wherein said source is a radio-frequency sourceand said voltmeter has a frequency response including the frequency ofsaid source.